This shows the user and PID, as well as information about how the file is being accessed (here, f, indicating an open file), and the command being used to access it (here, more).The -m switch shows all process accessing the filesystem, which the file is on; or you can simply specify a filesystem rather than a file. So fuser -m /dev/sdb1 output looks like:

/dev/sdb1: 18647c 16875m 7122c 19579m 1606c 7258c 5760c 13644c

Most of these processes are accessing the current directory (c), with a couple using shared libraries (m). Use the -v switch, to get more information.

To kill all processes accessing a particular file, use fuser -k filename.txt (add -i for a confirmation message before each kill). You can also use fuser on network ports: fuser -n tcp 21. This will show which process is using port 21 (useful if you get a ‘port busy’ message).